The present invention relates to a dual evaporator air conditioning system of a type useful in transportation vehicles and other applications.
Automotive air conditioning systems employ a compressor (typically driven by a combustion engine) and condenser mounted under the hood which are fluidically coupled to an HVAC case in the interior passenger compartment. The HVAC case includes a blower for circulating air and heat exchangers such as a heater core and an evaporator for conditioning the circulated air. In sedans and other vehicles having no more than two rows of seating, a single HVAC case is typically located behind the front instrument panel at a center line of the vehicle. Due to the relatively large size of the HVAC case, little space may be left behind the instrument panel for other components such as electronic accessories and storage bins. In order to convert the center stack area of the instrument panel to other uses, it is becoming desirable to break up the large HVAC case into two smaller HVAC cases to separately supply conditioned air at the driver side and the passenger side of the front instrument panel, respectively.
In larger vehicles with three or more seating rows, a single HVAC case may be unable to efficiently condition air to all areas of the vehicle. Furthermore, there has been a trend toward providing individually-controlled comfort functions in different zones or seating positions (e.g., giving rear seat passengers separate temperature and blower controls). This has led to the use of auxiliary HVAC cases disposed in a rear seating area, for example. Each separate HVAC case includes a respective blower and evaporator to separately cool the respective airflows according to separate demands.
The air conditioning compressor supplies liquefied refrigerant to a thermal expansion valve (TXV), which meters an amount of refrigerant to the evaporator that achieves a target evaporator temperature. The separate evaporators in a dual evaporator system may have different cooling demands placed on them at any particular time. In conventional systems, the amount of refrigerant metered to an evaporator is controlled such that all the refrigerant reaches a vapor state by the time it exits the evaporator. In other words, the liquefied refrigerant has a low vapor quality entering the evaporator and has a vapor quality of substantially 100% when leaving the evaporator to return to the compressor. In order to meet this condition in each separate evaporator of a dual evaporator system, the liquid refrigerant arriving from the condenser is usually split into two paths that are individually controlled. For example, in U.S. Pat. No. 6,983,793, primary and auxiliary HVAC units receive separate supplies of refrigerant through respective TXVs. Each TXV is separately controlled according to the temperature of the respective evaporator.
Due to the expense of the TXV, it would be desirable to meter refrigerant to both evaporators from a single TXV. As shown in U.S. Patent Application Publication 2007/0151287A1, two evaporators receive refrigerant flow in parallel from a pressure reducer and a distribution valve that controls the relative proportion of refrigerant sent to each evaporator. The distribution valve is controlled based on the cooling loads of the evaporators. Special control algorithms are required along with separate sensors for measuring the temperatures of each evaporator. Thus, it would be desirable to implement a dual evaporator system with a single TXV without requiring additional valves or modifications to the control system or algorithms.